Within human breast and colon carcinoma lesions, as well as established human tumor cell lines, there exists extensive heterogeneity in the expression of defined tumor antigens as recognized by the binding of monoclonal antibodies (MAbs). This heterogeneity of tumor antigen expression can be found within defined areas of the tumor and within subcellular compartments of a single tumor cell. These studies have shown that human tumor cells have the ability to intrinsically regulate the expression of the tumor antigens. Factors such as 1) cell cycle kinetics, 2) long-term growth in vitro, 3) clonal variability, and 4) growth of tumor cells in three-dimensional structures all alter the antigen phenotype of human tumor cells. The apparent ability of human tumor cells to intrinsically modulate their tumor antigen expression provides insight into the factors contributing to the extensive antigenic heterogeneity observed in these cell populations. Studies were conducted to identify those biological response modifiers that can override the intrinsic changes in antigen expression leading to an increase in the cell surface binding of monoclonal antibodies. Recombinant human leukocyte interferon (Hu-IFN-AlphaA) was found to increase the binding of specific MAbs to the surface of human breast and colon carcinoma cells in a dose-dependent manner. Utilizing 10-100 units Hu-IFN-AlphaA, tumor antigen expression can be dramatically increased independent of any change in tumor cell proliferation. Cell sorter analysis revealed that the Hu-IFN-AlphaA mediated increase in tumor antigen expression is a result of an increased amount of antigen per cell as well as the recruitment of previously antigen negative cells to become antigen positive. These studies may thus lead to: (i) a better definition of the transformed phenotype and (ii) the development of new strategies to enhance the in vivo binding of monoclonal antibodies to tumor antigens.